gistfile1.rb

# thanks to @judofyr for explaining this to me like thus:
# when ruby sees "bar =" (even thought it's not eval'd) it creates a local variable and assigns it to nil.
# so the first "bar" is a method call, while the second is a local variable. it should work if you replace
# it with "self.bar".

class Foo
  attr_accessor :bar, :id
  
  def initialize id
    @id = id
  end
  
  def test
    puts "bar was '#{bar}'"
    unless id
      raise "id is nil or false"
      bar = "this should not be"
    end
    puts "bar is now '#{bar}'"
  end
  
  def proper
    puts "bar was '#{@bar}'"
    unless id
      raise "id is nil or false" 
      @bar = "this should not be"
    end
    puts "bar is now '#{@bar}'"    
  end
end

foo = Foo.new(1)
foo.bar = "pants"
foo.test

puts "---"

foo.proper

#output:
#  bar was 'pants'
#  bar is now ''
#  ---
#  bar was 'pants'
#  bar is now 'pants'